/*
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you under the Apache License, Version 2.0 (the
 * "License"); you may not use this file except in compliance
 * with the License.  You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.apache.flink.table.planner.functions.aggfunctions;

import org.apache.flink.table.api.DataTypes;
import org.apache.flink.table.api.TableException;
import org.apache.flink.table.expressions.Expression;
import org.apache.flink.table.expressions.UnresolvedReferenceExpression;
import org.apache.flink.table.types.DataType;

import static org.apache.flink.table.expressions.ApiExpressionUtils.unresolvedRef;
import static org.apache.flink.table.planner.expressions.ExpressionBuilder.concat;
import static org.apache.flink.table.planner.expressions.ExpressionBuilder.ifThenElse;
import static org.apache.flink.table.planner.expressions.ExpressionBuilder.isNull;
import static org.apache.flink.table.planner.expressions.ExpressionBuilder.literal;
import static org.apache.flink.table.planner.expressions.ExpressionBuilder.nullOf;

/** built-in listagg aggregate function. */
public class ListAggFunction extends DeclarativeAggregateFunction {
    private int operandCount;
    private UnresolvedReferenceExpression acc = unresolvedRef("concatAcc");
    private UnresolvedReferenceExpression accDelimiter = unresolvedRef("accDelimiter");
    private Expression delimiter;
    private Expression operand;

    public ListAggFunction(int operandCount) {
        this.operandCount = operandCount;
        if (operandCount == 1) {
            delimiter = literal(",", DataTypes.STRING().notNull());
            operand = operand(0);
        } else {
            delimiter = operand(1);
            operand = operand(0);
        }
    }

    @Override
    public int operandCount() {
        return operandCount;
    }

    @Override
    public UnresolvedReferenceExpression[] aggBufferAttributes() {
        return new UnresolvedReferenceExpression[] {accDelimiter, acc};
    }

    @Override
    public DataType[] getAggBufferTypes() {
        return new DataType[] {DataTypes.STRING(), DataTypes.STRING()};
    }

    @Override
    public DataType getResultType() {
        return DataTypes.STRING();
    }

    @Override
    public Expression[] initialValuesExpressions() {
        return new Expression[] {
            /* delimiter */ literal(",", DataTypes.STRING().notNull()),
            /* acc */ nullOf(DataTypes.STRING())
        };
    }

    @Override
    public Expression[] accumulateExpressions() {
        return new Expression[] {
            /* delimiter */
            delimiter,
            /* acc */
            ifThenElse(
                    isNull(operand),
                    acc,
                    ifThenElse(isNull(acc), operand, concat(concat(acc, delimiter), operand)))
        };
    }

    @Override
    public Expression[] retractExpressions() {
        throw new TableException("This function does not support retraction.");
    }

    @Override
    public Expression[] mergeExpressions() {
        return new Expression[] {
            /* delimiter */
            mergeOperand(accDelimiter),
            /* acc */
            ifThenElse(
                    isNull(mergeOperand(acc)),
                    acc,
                    ifThenElse(
                            isNull(acc),
                            mergeOperand(acc),
                            concat(concat(acc, mergeOperand(accDelimiter)), mergeOperand(acc))))
        };
    }

    @Override
    public Expression getValueExpression() {
        return acc;
    }
}
